There are two kinds of contact lenses, soft and hard. The material for the hard contact lens(HCL) is a synthetic resin with network structure containing poly(methyl methacrylate)(PMMA) as a main component and a crosslinking agent. This polymer is widely used in clinical field for HCL because the transparency, the molding processability, the biostability, and the durability are excellent, but this HCL is not suitable for extended wear because the oxygen permeability is not high enough for the respiratory metabolism of the eyeballs and the hard nature of the material gives a feeling of foreign body to the patient.
Materials widely used for the soft contact lens(SCL) in clinical field include a silicone elastomer and a copolymer of butyl acrylate(BA) and butyl methacrylate(BMA) as hydrophobic ones and poly(hydroxyethyl methacrylate)(PHEMA) and a copolymer of methyl methacrylate(MMA) and vinyl pyrrolidone(VP) as hydrophilic ones. These materials are superior to those of the HCL in oxygen permeability. However, the hydrophobic materials have disadvantages such as poor wear feeling caused by lower water wettability, remarkable adsorption of lipids, and surface damage due to high electrostatic charging, while the hydrophilic ones are very high in production cost, strongly adsorbed with proteins, and very low in mechanical strength, although they give excellent wear feeling and minimum damage to cornea.
The PVA hydrogels have already been proposed as materials for SCL, for instance PVA gels crosslinked with glyoxal (U.S. Pat. No. 3,408,429) and PVA contact lens crosslinked with borates (Japanese Unexamined Patent Publication (Tokkyo Kokai) No. 75111/1983). However, these may stimulate the patient cornea through unreacted compounds remaining in the gel or degradation products with the lapse of time, since these gels are chemically crosslinked with glyoxal and borates. On the other hand, a PVA contact lens is proposed which is prepared through annealing (Japanese Unexamined Patent Publication (Tokkyo Kokai) No. 214128/1982), but this is not sufficient in water content and mechanical strength and needs a complicated and long fabrication process.
As a result of the present inventors' intensive study for solving the defects of the conventional PVA hydrogel SCL, and for developing a simple and inexpensive method of production of SCL which is improved in mechanical strength and has a high water content, an SCL of a high strength, water content and oxygen permeability fabricated in a simple and inexpensive method can be obtained by dissolving PVA in a mixed solvent consisting of water and an organic solvent, followed by crystallizing the PVA between its molecular chains at a temperature of not more than ordinary temperature.